This invention relates to the manufacture of seamless tubes of polymers, such as polyimides.
The fabrication of a seamless polyimide tube has been discussed in patent literature in recent years. U.S. Pat. No. 5,433,913 to Kawauchi et al. serves as an example. It employs a ring coating process coupled with ultrahigh viscosity coating solutions.
High performance polyimides are very difficult to process. They are thermosetting resins, which cannot be reformed with heat. Furthermore, they are soluble in a limited number of relatively high-boiling solvents, such as n-methyl-2-pyrrolidone (NMP) and dimethyl acetamine] (DMAc). Curing conditions for polyimides are difficult to optimize and have a profound effect on the mechanical properties of the cured film. Additionally, the high temperatures required for polyimide curing limit the choice of coating substrates to metal and ceramics. Finally, the cost of polyamic acid resins is high, giving great advantage to a process which can minimize resin waste.
This invention is a process by which a seamless polyimide tube can be cast from a high viscosity polyamic acid resin. The process is simple, effective, relatively fast, and highly efficient in that minimal waste material is generated.
In accordance with this invention, the amount of resin to constitute the final tube is computed and an amount of solution having that amount precursor for that amount of resin is defined. That amount of solution is applied generally evenly over a mandrel. The solution may be forced from a nozzle or simply poured. The mandrel is rotated while a doctor blade, canted to keep material form of the doctor blade, is moved along the length of the mandrel at a amount of separation from the mandrel consistent with the predetermined thickness which the final tube is to have. After the doctoring step, the solvent is extracted with heat. The film is further cured to form the polyimide resin and then removed from the mandrel.